The letter is a formality on two counts: the supervisors have the votes to override Newsom's veto, and the mayor has said he'll ignore the legislation anyway.

At issue is legislation, written by Supervisor David Campos, requiring that undocumented youth be reported to federal immigration officials for possible deportation after they're convicted of a felony, not when they're first arrested, as is the current policy.

Newsom, has said the legislation must be ignored because it violates federal law.

"The sanctuary ordinance as originally conceived and adopted was designed to protect those residents of our city who are law-abiding," Newsom wrote. "It was never meant to serve as a shield for people accused of committing serious crimes."

Newsom's letter also quotes a memorandum from the city attorney's office advising that "there is a serious risk" that if the matter is brought to court, a judge would strike down not only the Campos legislation but "possibly the entire sanctuary city ordinance."

Campos said, "It's a sad day for San Francisco. Our mayor has chosen to be on the wrong side of history on this issue."